1. Field of the Invention
This invention relates in general to telephone communications. In particular, the invention relates to an improved technique for the detection of signaling tones, such as those associated with caller identification services, transmitted over a voice communications link.
2. Background Art
In recent years, numerous advanced telecommunications features have become readily available to home and business telephone users. One such feature that has attained great popularity is caller identification, often referred to as “caller ID” or “CID”. Conventional caller ID systems typically involve the transmission of data identifying or corresponding to the calling party's telephone number and/or name from the telephone company central office switch to a called party's CID-capable telephone set or CID box. This data is transmitted over the telephone line during the “silent” period following the first ring of the called party's telephone set. The called party is thus able to determine the identity of the calling party before answering the incoming call. Additionally, if the called party is unavailable or elects not to answer the incoming call, a record of calls received can be stored locally in the memory of the CID-capable telephone set or CID box for later reference by the user. Such conventional caller ID service only operates when the called party's phone is “on hook”, i.e. when the called party is not engaged in a telephone conversation.
Conventional caller ID has been further improved through the implementation of a system known as Type II caller ID. Type II CID expands upon the functionality of conventional CID by combining conventional caller ID with a “call waiting” service. A call waiting service operates during an ongoing telephone conversation by audibly signaling a called party to indicate that a second calling party has placed a call to the called party's phone. The called party upon hearing a signal tone can elect to put the first call on hold and establish a connection with the second calling party. After having completed talking to the second calling party, the called party can reestablish a connection with and resume talking to the first party.
However, unlike conventional CID, Type II CID also operates while a called party is already engaged in a telephone conversation and receives a telephone call from a second party. The system first notifies the user of the incoming call by transmitting an audible “call waiting” tone, and then proceeds to transmit Type II CID data to the called party thereby identifying the second calling party. Just as in traditional caller ID, the new caller's information is displayed on the Type II CID-capable telephone set or CID box. Thus, the user is able to determine the relative importance of a newly received call, and to decide whether to interrupt the call in which the user is already engaged by switching over to the second call.
Several standards have been developed for the implementation of Type II CID, such as the BellCore GR-3004 specification, or the BT 202 specification. Such standards dictate that Type II CID operation is initiated via the transmission of a specific signal over the telephone line during the telephone conversation. This signal is known as the Customer Premises Equipment Alerting Signal (“CAS”). According to the BellCore specification, the CAS is a dual-tone signal consisting of a 2130 Hz tone and a 2750 Hz tone, sometimes called CAS tone A and CAS tone B, respectively. When the CAS is detected by the Type II CID-capable customer premises equipment (“CPE”) such as a cordless telephone set, the CPE initiates a brief digital communications link with the central office by acknowledging receipt and recognition of the CAS by the CPE. This in turn causes the central office to transmit the modulated digital CID data that is then received by the called party's CPE and displayed on the Type II CID-capable telephone set or CID box display.
To avoid interrupting the user's telephone conversation with an unpleasant burst of noise when the digital communications occur, the CPE mutes the user's audio signal as soon as the CAS is detected. The CID data is then transmitted during the muted period in a brief burst of digital data, after which the caller can continue the current conversation experiencing minimal interruption. If the transmitted CAS tones are not detected by the CPE due to the calling party's telephone set not being Type II enabled, the CPE will not send the requisite acknowledgement signal, and the system will not transmit the CID information to the user. This CPE acknowledgement feature prevents users of CPE without Type II CID functionality from being subjected to a loud burst of digital noise while the CID data is transmitted unnecessarily each time a call waiting signal is received. However, it is also possible that Type II CID-equipped CPE can nevertheless fail to detect a valid CAS, thereby depriving the user of the Type II CID functionality. Furthermore, if the CPE detects CAS tones when a CAS has not in fact been sent by the central office (a “false” CAS detection), the user's telephone conversation is unnecessarily interrupted while the CPE mutes the audio signal to attempt digital communications with the central office. Thus, crucial to the effective implementation of Type II CID is the reliable detection of CAS tones received from the central office, as well as the reliable rejection of false CAS tones.
This CAS detection task is often particularly difficult because the CAS is transmitted over the same channel on which the telephonic audio communications are taking place, at the same time during which these same communications are taking place, and within the same limited frequency bandwidth supported by modern telephone networks. Many techniques may be used by the CPE to detect dual tone frequencies such as a CAS, including analog bandpass filtering followed by a peak detector or PLL, or digital techniques that use digital filtering or discrete Fourier transforms (DTF). Additionally, several techniques are known which are specifically intended to improve the reliability of CAS detection. Such techniques often involve setting stricter standards for the detected CAS signal that must be satisfied before the CAS will be considered to be genuine. These include setting tight criteria for characteristics such as the signal duration, the difference in signal levels between tones in the dual-tone CAS, the allowable frequency of each CAS tone, or the signal level of each tone with respect to the remainder of the voiceband frequencies or some subset thereof. However, as the tightness of such criteria is increased, the system becomes increasingly likely to fail to identify a genuine CAS tone due to uncontrollable variations in the CAS signaling and noise and distortion within the communications channel. Such failures to detect the Type II CID CAS are highly undesirable.
One technique to improve CAS detection reliability is set forth in U.S. Pat. No. 6,122,353 issued to Brady et al., which discloses a technique whereby the CPE's audio signal is muted or disconnected altogether from the transmission channel as soon a potential CAS is detected. The CPE then continues to monitor the CAS on the telephone line for its remaining duration while the near-end audio input remains muted. If the output of the CAS detection mechanism continues to indicate the presence of a CAS even absent the near-end audio (which has been removed due to the audio signal being disconnected), then the incoming signal is likely to be a genuine CAS being received from the central office rather than a spurious product of the audio signal produced by the near-end telephone. However, this technique fails to eliminate interruptions caused by false CAS detections, because the near-end audio must still be muted. Rather, it only decreases the disruption (i.e. decreases the duration of the erroneous muting period) caused by the false CAS detections originating from near-end voice signals, while failing to eliminate false CAS detections caused by voice signals coming from the calling party.
Another method to improve CAS detection reliability involves measuring the energy of the CAS and comparing that energy to the overall energy present in the voiceband channel. If a valid CAS were present, then there would likely be a significant difference in energies between the CAS energy level and the energy level of the remainder of the channel. If it is determined that the energy across the voiceband channel is roughly the same as, or within a certain margin of, the measured CAS, then the CAS would be rejected as a probable by-product of complex audio signals on the telephone line. Unfortunately, this technique very often cannot distinguish between valid but speech-corrupted CAS tones and signals with high energy levels at the CAS tone frequencies due instead to overall very high levels of audio energy across the voiceband channel, thereby resulting in the occasional failure to properly deliver the Type II CID data.
Another technique is disclosed by U.S. Pat. No. 5,519,774, issued to Battista et al., in which select sampled bandwidths are chosen specifically from the upper portion of the voiceband, between the frequencies of 2000 and 3000 Hz in which energy from typical voice signals is likely to be low. The measured energies are then weighted by an experimentally-derived weighting function using a complex adaptive algorithm and compared to the energy levels measured at the CAS frequencies to determine whether a detected CAS is valid. However, this technique requires an iterative experimental procedure and prior knowledge of false CAS detections to optimize the complex adaptive algorithm that implements the technique and obtain good performance. Also, the technique's high level of complexity adds to the cost, size and power consumption of products that implement it.
Thus, it is desirable and an object of the invention to provide simple and comparatively inexpensive technique for the reliable detection of CAS signaling.